U.S. patent application number 11/176866 was filed with the patent office on 2007-01-11 for method of realizing direct bonding between metal wires and copper pads by means of thermosonic wire bonding using shielding gas spraying device.
Invention is credited to Jong-Ning Aoh, Cheng-Li Chuang.
Application Number | 20070010083 11/176866 |
Document ID | / |
Family ID | 37618811 |
Filed Date | 2007-01-11 |
United States Patent
Application |
20070010083 |
Kind Code |
A1 |
Aoh; Jong-Ning ; et
al. |
January 11, 2007 |
Method of realizing direct bonding between metal wires and copper
pads by means of thermosonic wire bonding using shielding gas
spraying device
Abstract
Discloses is a method of realizing direct bonding between metal
wires and copper pads by means of thermosonic wire bonding using a
shielding gas spraying device, where a shielding gas is provided
between metal wires and a chip with copper pads during thermosonic
wire bonding to form a gas shielding zone around a wire bonding
zone of a thermosonic wire bonder, to effectively prevent
oxidization of the chips with copper pads at elevated temperature
of a heating stage of the thermosonic wire bonder; thus, the
provision of the shielding gas spraying device according to this
invention allows the metal wires to be directly bonded to the chips
with copper pads by means of thermosonic wire bonding process, and
overcomes the problem caused by the copper oxide film, thereby
meeting the demand of high bondability required by the
semiconductor packaging industry, and significantly improving the
bonding strength and reliability of the metal wires and chip with
copper pads.
Inventors: |
Aoh; Jong-Ning; (Min-Hsiung,
TW) ; Chuang; Cheng-Li; (Min-Hsiung, TW) |
Correspondence
Address: |
SCHMEISER, OLSEN & WATTS
22 CENTURY HILL DRIVE
SUITE 302
LATHAM
NY
12110
US
|
Family ID: |
37618811 |
Appl. No.: |
11/176866 |
Filed: |
July 7, 2005 |
Current U.S.
Class: |
438/612 ;
257/E21.518; 257/E23.02; 438/617 |
Current CPC
Class: |
H01L 2224/85075
20130101; H01L 24/02 20130101; H01L 2224/451 20130101; H01L
2924/01074 20130101; H01L 24/48 20130101; H01L 2224/78 20130101;
H01L 2224/48247 20130101; H01L 2924/01047 20130101; H01L 2224/48463
20130101; B23K 2101/40 20180801; H01L 2924/01006 20130101; H01L
2224/85207 20130101; H01L 2924/3025 20130101; H01L 2924/01007
20130101; H01L 24/45 20130101; H01L 2924/30105 20130101; B23K
20/004 20130101; H01L 2924/01018 20130101; H01L 2924/01029
20130101; B23K 20/14 20130101; H01L 24/85 20130101; H01L 2224/05647
20130101; H01L 2224/04042 20130101; H01L 2224/48091 20130101; H01L
2224/85065 20130101; H01L 2924/01013 20130101; H01L 24/05 20130101;
H01L 24/78 20130101; H01L 2224/85065 20130101; H01L 2924/01001
20130101; H01L 2224/85075 20130101; H01L 2924/01018 20130101; H01L
2224/85075 20130101; H01L 2924/01007 20130101; H01L 2224/85075
20130101; H01L 2924/01006 20130101; H01L 2924/01008 20130101; H01L
2224/85075 20130101; H01L 2924/01002 20130101; H01L 2224/48463
20130101; H01L 2924/00014 20130101; H01L 2224/48091 20130101; H01L
2924/00014 20130101; H01L 2224/05647 20130101; H01L 2924/00014
20130101; H01L 2224/451 20130101; H01L 2924/00014 20130101; H01L
2224/451 20130101; H01L 2924/00015 20130101 |
Class at
Publication: |
438/612 ;
438/617 |
International
Class: |
H01L 21/44 20060101
H01L021/44 |
Claims
1. A method of realizing direct bonding between metal wires and
copper pads by means of thermosonic wire bonding using a shielding
gas spraying device, comprising the steps of: providing a shielding
gas spraying device during thermosonic wire bonding between metal
wires and chips with copper pads; and providing a shielding gas to
shield a surface of the chip with copper pads, wherein the
shielding gas forms a shielding zone above the chip with copper
pads to prevent air from entering the shielding zone and surfaces
of copper pads from oxidization, so as to allow direct thermosonic
bonding between metal wires and chips with copper pads.
2. The method of realizing bonding between metal wires and copper
pads by means of thermosonic wire bonding using a shielding gas
spraying device of claim 1, wherein: the shielding gas browse forms
a shielding range in a form of a dome or an air curtain over the
surface of the chip with copper pads; the shielding gas spraying
device includes a control valve for controlling gas supply and flow
rate, the control valve acts synchronously with a leadframe feeding
and unloading system having a leadframe feeding device and a
leadframe unloading device; whereby when the leadframe feeding
device of the thermosonic wire bonds is activated, the control
valve is activated at the same time to supply the shielding gas to
shield the chip with copper pads and to prevent oxidization of
copper pads; and when the leadframe unloading device is activated
upon completion of wire bonding, the control valve is switched off
to cut the shielding gas supply.
3. The method of realizing direct bonding between metal wires and
copper pads by means of thermosonic wire bonding using a shielding
gas spraying device of claim 1, wherein the spraying device has an
adjustable shielding range, capable of adjusting height and
orientation of the shielding gas to accommodate chip specifications
and chip size.
4. The method of realizing direct bonding between metal wires and
copper pads by means of thermosonic wire bonding using a shielding
gas spraying device of claim 1, wherein the shielding gas is
selected from inert gases and reduction atmospheric gases,
including: helium (He), argon (Ar), Nitrogen (N.sub.2), Hydrogen
(H.sub.2), Carbon Dioxide (CO.sub.2), or a gaseous combination of
any two of the said gases of any proportion.
Description
FIELD OF INVENTION
[0001] This invention relates to a method of realizing direct
bonding between metal wires and copper pads by means of thermosonic
wire bonding using a shielding gas spraying device, where a
shielding gas spraying device is employed to realize the direct
bonding process between metal wires and copper pads by means of
thermosonic wire bonding.
BACKGROUND
[0002] Along with the miniaturization of components and quick
development of the process of manufacturing semiconductors to
submicron level, the component density per unit area increases
dramatically such that the reduced interconnects dimension between
the components induces higher electric resistance and the narrow
line widths increase the parasitic capacitance thereby resulting in
significant signal delays. With the inevitable trend of
miniaturization, an optimum measure of reducing the signal delays
is to employ dielectric materials with a low dielectric constant
and replace the conventional aluminum interconnects with copper of
better electrical conductivity. However, the introduction of copper
interconnects have resulted in significant impacts in the packaging
process due to the entire different mechanisms involved in the
oxidization of copper and aluminum metals. For aluminum, a
self-passivation is easily formed on its surface to prevent
oxidation of the aluminum underneath the self-passivation layer,
where the aluminum oxide film is scrubbed off by ultrasonic power
during the thermosonic wire bonding process so as to ensure bonding
between the metal wires and the aluminum pads. Copper, on the
contrary, is easily oxidized under the atmosphere, since the copper
oxide film does not provide the same self-passivation effect as the
aluminum oxide film does, such that the copper oxide of the copper
pads film would continue to grow on the surfaces of the copper
pads. Not only does such a copper oxide film on the copper pads
become a bottleneck in the thermosonic wire bonding process, it
adversely affects the mechanical and physical properties of copper
pads. The thermosonic wire bonding process is widely employed in
the packaging industry. The underlying principles involved in the
thermosonic wire bonding process include ultrasonic bonding and
thermal compression bonding, where a heating stage provides heat
that serves as the activation energy required for the atomic
inter-diffusion bonding between the metal wires and metal pads, as
well as ultrasonic power that results in a temperature rise at the
bonding interface due to friction between the wires and metal pads
to facilitate atomic inter-diffusion between the metal pads and
metal wires thereby forming good ball bond or stitch bonds. Thus,
in a thermosonic wire bonding process, the temperature required for
the heating stage during the thermal compression bonding should be
lower enough to prevent the semiconductor components from thermal
damages. Generally speaking, the temperature of the heating stage
required for thermosonic wire bonding is set within the range of
120.degree. C. to 220.degree. C. The appropriate bonding
temperature range (120.degree. C. to 220.degree. C.) for a
thermosonic wire bonder, however, does not allow a problem-free
bonding between the metal wires and the chip with copper pads, as
the surfaces of copper pads under such a temperature range are
easily oxidized and the thickness of the oxide film increases with
increasing stage temperature, as shown in FIG. 1, wherein the
copper oxide film on the copper pads significantly affects the
bondability and bonding strength of the thermosonic bonds. The
oxide film becomes a barrier for the atomic inter-diffusion at
wires copper pads interface and hinders the wires from bonded to
copper pads. The copper oxide film on the copper pads surface
cannot serve as a self-passivation layer to prevent oxidation of
the parent metal underneath the oxide layer as the aluminum oxide
film can, and the copper oxide film on the copper pads cannot be
scrubbed off by the ultrasonic power generated by the thermosonic
wire bonder, whereby the copper oxide film becomes a barrier for
the bonding of metal wires copper pads and the oxidation of copper
of is a serious bottleneck in the thermosonic wire bonding process
for bonding metal wires to copper pads. A development for new
technology capable of preventing or reducing the oxidization of the
copper pads in the thermosonic wire bonding process, and overcoming
the problem caused by the copper oxide film to allow a direct
bonding of metal wires to copper pads in the thermosonic wire
bonding process is thus essential, thereby meeting the demand of
high bondability required by the semiconductor packaging industry,
ensuring the superior performance of the chips with copper
interconnects, and significantly improving the quality and
reliability of the thermosonic bonding of metal wires and chips
with copper pads.
SUMMARY OF INVENTION
[0003] In view of the shortcomings of the thermosonic wire bonding
process adopted in the conventional semiconductor components, the
invention discloses a method of realizing direct bonding between
metal wires and copper pads by means of thermosonic wire bonding
using a shielding gas spraying device, where a shielding gas
spraying device is employed to realize the direct bonding between
metal wires and copper pads by means of thermosonic wire
bonding.
[0004] Thus, it is a primary objective of this invention to provide
a method of realizing direct bonding between metal wires and copper
pads by means of thermosonic wire bonding using a shielding gas
spraying device that would prevent oxidization of copper pads,
allow direct bonding of metal wires to chips with copper pads, and
ensure superior performance of the chips with copper pads.
[0005] It is another objective of this invention to provide a
method of realizing direct bonding between metal wires and copper
pads by means of thermosonic wire bonding using a shielding gas
spraying device, where a shielding gas spraying device is able to
adjust the gas-spraying orientation in accordance with the chip
dimensions, such that the device may meet the demands of various
chip sizes without affecting the handling space required for manual
wire-inserting operation performed by the operators.
[0006] It is another objective of this invention to provide a
method of realizing direct bonding between metal wires and copper
pads by means of thermosonic wire bonding using a shielding gas
spraying device, where the metal wires are directly bonded to chips
with copper pads at high bondability meeting the industry standard
while improving the bonding strength and reliability during the
thermosonic wire bonding process between copper metal wires and
chip with copper pads.
[0007] To achieve the above objectives, this invention discloses a
method of realizing direct bonding between metal wires and copper
pads by means of thermosonic wire bonding using a shielding gas
spraying device, comprising the steps of: providing a shielding gas
spraying device during the thermosonic wire bonding between copper
metal wires and chips with copper pads; providing a shielding gas
to shield the surface of the chip with copper pads, wherein the
shielding gas forms a shielding zone above the chip with copper
pads to prevent air from entering the shielding zone and the
surfaces of copper pads from oxidization, so as to allow direct
bonding between metal wires and chip with copper pads.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] These and other modifications and advantages will become
even more apparent from the following detailed description of a
preferred embodiment of the invention and from the drawings in
which:
[0009] FIG. 1 is a plot showing the relationship between the
thickness of the oxide film growing on a chip with copper pads and
the temperature of a heating stage;
[0010] FIG. 2 is a schematic view illustrating the components for
spraying a shielding gas according to this invention;
[0011] FIG. 3a and FIG. 3b include schematic views illustrating a
shield gas browse forming a shield in the form of a dome and an air
curtain over the surfaces of copper pads;
[0012] FIG. 4 illustrates the distribution profiles of the amount
of oxygen atoms along the depth within the surfaces of the copper
pads after curing;
[0013] FIG. 5 is a SEM micrograph showing successful thermosonic
bonding of metal wires to the chip with copper pads according to
this invention;
[0014] FIG. 6 is a SEM micrograph showing failed thermosonic
bonding of metal wires to the chip with copper pads under the air
atmosphere;
[0015] FIG. 7 is a comparative plot showing the relationships
between the bondability of the bonding between metal wires and
chips with copper pads and the temperature of the heating stage
under inert gas shielded conditions according to this invention,
and under those at the atmosphere;
[0016] FIG. 8 is a comparative plot showing the relationships
between the bonding strength of metal wires to chips with copper
pads and the temperature of the heating stage under inert gas
shielded conditions according to this invention, and under those at
the atmosphere;
[0017] FIG. 9 illustrates the distribution profiles of the amount
of oxygen atoms along the depth within the surfaces of the copper
pads where the chips with copper pads are shielded by argon
according to this invention after heated on a heating stage;
[0018] FIG. 10 illustrates the distribution profiles of the amount
of oxygen atoms along the depth within the surfaces of the copper
pads where the chips with copper pads are heated on a heating stage
under the atmosphere.
DETAILED DESCRIPTION OF THE INVENTION (PREFERRED EMBODIMENTS)
[0019] According to this invention, the process of thermosonic wire
bonding between metal wires and chips with copper pads, the chips
with copper pads located at a wire bonding zone on the heating
stage is shielded by a inert gas shielding browse to prevent the
surfaces of copper pads on the chips from oxidizing due to heating
during thermosonic wire bonding, such that metal wires may be
directly bonded to the copper pads by means of thermosonic bonding.
Not only is this invention an effective means for thermosonic wire
bonding of metal wires to chips with copper pads which meets the
demand of perfect bondability required by the semiconductor
packaging industry, but also ensure superior performance of the
chips with copper pads, improve the bonding strength between the
metal wires and copper pads, and enhance the reliability of the
thermosonic wire bonding process for bonding of metal wires to
chips with copper pads. In a shielding gas spraying device of this
invention, as shown in FIG. 2, a shield consisting of a shielding
gas browse forms a shielding zone in the form of a dome (FIG. 3a)
or an air curtain (FIG. 3b) over the surfaces of chips with copper
pads. However, these embodiments do not intend to limit the
shielding configurations as shown in FIG. 3a and FIG. 3b. The
shielding gas spraying device includes a shielding gas control
valve 21 for controlling the flow rate, on/off of the shielding gas
spray. The control valve 21 acts synchronously with a leadframe
feeding device of a thermosonic wire bonder. When the leadframe
feeding device of the wire bonder is activated to feed a leadframe
onto the heating stage, the control valve 21 is activated at the
same time to supply the shielding gas to shield the copper pads and
to prevent the copper pads from oxidizing. After all copper pads on
the leadframe have been subjected to the thermosonic wire bonding
process, the leadframe unloading device is then activated and the
control signal is transmitted to the shielding gas control valve 21
at the same time to switch off the control valve 21, preventing
unnecessary consumption of the shielding gas. This control module
effectively reduces the consumption of the shielding gas. The
shielding gas spraying device further includes a shielding gas
shielding range adjusting device 27 for adjusting height and
orientation of the shielding gas browse to accommodate chips with
copper pads of different specifications and dimensions, such that
the device according to this invention can be applied to all kinds
of chips with copper pads. Since broken or unsuccessful bonds are
sometimes found after an automatic thermosonic wire bonding process
is conducted, such that a manual wire-inserting operation may be
required, the shielding gas spraying device of this invention
should not interfere the handling space required for manual
wire-inserting operation performed by the operators. That is, the
operation efficiency of the thermosonic wire bonder would not be
affected with the implementation of this invention.
[0020] The following embodiments that adopt the process and the
chips with copper pads of this invention by subjecting the metal
wires and chips with copper pads to the thermosonic wire bonding
process, are discussed to show the effectiveness of this invention,
but do not intend to limit the scope of the invention.
[0021] Chips with copper pads are mounted on a leadframe, with a
wafer dimensioned to 6 mm.times.6 mm. The leadframe is made of
copper alloy C7025. A second stitch bond is deposited on a surface
thereof with a silver film to improve the stitch bond quality of
the second thermosonic stitch bond of the metal wires. The model
number of the leadframe is 128-362.times.362, where each leadframe
includes six chips and each chip includes 128 copper pads. To
prevent rapid growth of copper oxide film on the surfaces of the
copper pads during die sawing and die mounting of the wafer, the
chips are shielded by nitrogen for 30 minutes at 150.degree. C.
during curing in the die mounting process. To ensure accurate
experimental results, an Auger electron spectrometer (AES) is used
to analyze the surface of the chips with copper pads mounted on the
leadframe, and to detect the distribution profiles of the amount of
oxygen atoms along the depth within the surfaces of the copper
pads, as shown in FIG. 4. It is found that the surfaces of copper
pads contain only minimum amount of oxygen, which reveals a minimum
degree of oxidization on the surfaces of copper pads so as not to
cause failure of the subsequent thermosonic wire bonding. The
shielding gas employed in this embodiment is commercially available
argon (99.99%) with very high purity. Because the specific weight
of argon is 23% higher than air, the argon is able to effectively
prevent air from entering the argon shielding zone. The shielding
gas spraying device as adopted in this embodiment is illustrated in
FIG. 2. The parameters employed in the thermosonic wire bonding
process are as shown in Table I. TABLE-US-00001 TABLE I Ultrasonic
power 0.15 W (with a scale configured to 100 on the control panel)
Bonding load 0.5 N Bonding time 20 ms Temperature of heating stage
90-220.degree. C. Metal wires diameter 25 .mu.m
[0022] In a thermosonic wire bonding process where the copper pads
are not subjected to a shielding gas, the samples as selected and
the parameters of the thermosonic wire bonding are identical to
those where the copper pads are subjected to a shielding gas. FIGS.
5 and 6 show the SEM micrographs of a successful thermosonic wire
bonding of metal wires to the chip with copper pads according to
this invention and a failed example of thermosonic bonding of metal
wires to the chip with copper pad under the atmosphere
respectively. It is known from FIG. 5 that, by using the shielding
gas spraying device of this invention, 100% of the metal wires are
directly bonded to the copper pads. On the contrary, in the case
where a shielding gas spraying device is not employed, most of the
metal wires failed to be bonded to the copper pads. Since the first
bond on copper pads failed, metal wires are subsequently brought
and bonded to leadframe by the bonding capillary. If the
temperature of the heating stage is varied from 90.degree. C. to
220.degree. C. and all others parameters remain unchanged, the
bondability and the bonding strength of metal wires to the chips
with copper pads is significantly influenced by the temperature, as
shown in FIGS. 7 and 8. In the case where a shielding gas is
employed, the bondability reaches 100% at higher temperature range
from 180.degree. C.-220.degree. C. On the other hand, in the case
where no shielding gas is employed, bondability decreases with
increasing temperature in the temperature range between 180.degree.
C. and 220.degree. C. Within such a high temperature range, the
surfaces of copper pads oxidize seriously when not shielded by the
shielding gas, whereby the copper oxide film grows rapidly, causing
failure of bonding between metal wires and the copper pads. FIG. 8
illustrates the relationship between bonding strength of the stitch
bonds to copper pads and the stage temperature where no shielding
gas is employed. Since the surfaces of copper pads oxidize and the
copper oxide film grows rapidly, thereby resulting in a poor
bonding strength between the metal wires and copper pads, which is
far below the value required by the industry specifications [1]. On
the other hand, in the case where shielding gas is employed, the
bonding strength exceeds far beyond the requirements in the
industry standards and increases with increasing stage temperature.
This is due to the fact that a higher heating temperature provides
higher activation energy to facilitate the inter-diffusion bonding
between metal wires and copper pads. To show the effectiveness of
shielding provided by this invention in preventing copper pads from
oxidization, chips with copper pads are placed on the heating stage
of the wire bonder for 2 minutes at 220.degree. C., where a
shielding gas is employed to shield the copper pads in one group,
while the other group of chips is heated under the atmosphere. An
AES is used to analyze the two groups of chips with copper pads,
and to detect the distribution profiles of the amount of oxygen
atoms along the depth within the surfaces of copper pads, as shown
in FIGS. 9 and 10, respectively. Upon comparing the oxygen atom
distribution profiles of chips with copper pads that are placed on
the heating stage of the thermosonic wire bonder with those of the
chips with copper pads right after curing (FIG. 4), one can easily
observe that the oxygen atom depth profiles for chips shielded by
the shielding gas, and those for chips right after curing are
extremely similar. Such a result indicates that the gas shielding
is able to effectively prevent the copper pads from oxidation
during heating. On the other hand, the amount of the oxygen atoms
found within the surfaces of the copper pads that are heated under
the atmosphere is higher than that found within the surfaces copper
pad right after curing. That is, the copper oxide film on the
surfaces of the chips with copper pads grows rapidly under the
atmosphere, causing failure in bonding the metal wires to the
copper pads. The afore-described examples show that, not only can
the shielding gas spraying device of this invention allows direct
bonding of the metal wires to the chips with copper pads, but also
effectively improve the bondability and bonding strength between
the metal wires and chips with copper pads, and enhance the
reliability of the thermosonic wire bonding process for metal wires
and chips with copper pads.
[0023] The present invention has been described with a preferred
embodiment thereof and it is understood that the scope and the
spirit of the invention as defined by the appended claims.
* * * * *